Canon EOS 1D Mark II N Instruction Manual page 81

Focusing an Off-Center Subject
To focus a peripheral subject not covered by the Area AF ellipse, follow
the procedure below. This technique is called focus lock.
Focus lock works only in the One-Shot AF mode.
1
Aim the Area AF or AF point over the subject and press
the shutter button halfway to focus.
2
Keep pressing the shutter button halfway and
recompose the picture as desired.
3
Take the picture.
If focus cannot be achieved, the focus confirmation light <
viewfinder will blink. If this occurs in the One-Shot AF mode, the picture
cannot be taken even if the shutter button is pressed fully. Recompose
the picture and try to focus again. Or see "When Autofocus Fails (Manual
Focusing)" (p.89).
In the AI Servo AF mode when the AF point selection is automatic, the
camera first uses the center AF point to focus. During autofocusing, if the
subject moves away from the center AF point, focus tracking continues
as long as the subject is covered by the Area AF ellipse. The active AF
point does not light.
In the AI Servo AF mode, when focus is achieved, the viewfinder's focus
confirmation light does not light and the beeper does not sound.
Selecting the AF Mode
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